Cracked / Chipped Piston

[robertm]

After two years and eight months of ownership and happy motoring, I'm finally starting my own troubleshooting thread.
Two weeks ago I was on a spirited club ride that traveled roads up to 10,000 feet above sea level. I didn't experience any trouble on the club ride but on my way to the car show the next day as part of a weekend two day event I started having a rough idle and misfiring throughout the rev range.

After limping the car home I found the number three spark plug ground electrode bent down and touching the positive electrode.
After a look inside the cylinder bore I noticed the edge of the piston had chipped off. I didn't see any parts loose in the cylinder so I wonder if the offending foreign/lose object escaped out of the exhaust valve.?

I am now trying to figure out what caused the piston to fail.

I had switched to a K&N filter about a year and a half ago but I don't think that would cause a lean condition while still using the factory air filter box, factory ADE jet needle, factory intake/exhaust manifold and factory exhaust.

I had never checked the timing even though Richard had advised me to advance it for higher altitude ie. more than the factory setting at sea level.
In my quest for answers I bought a timing light to check and it shows 6* advance at idle and the Haynes Manual shows it should be at 8*.
One thing to note, it is a 1981 998cc and has a 65DM4 electronic ignition distributor that a previous owner installed and the setting for that dizzy on a 1275cc shows 5*+1

My last thought is just old age fatigue.

But I am thinking the biggest factor could be that the timing was off/not advanced enough for my location.


For now my plan is to pull the engine and repair/rebuild it over the winter.
My mileage is at 72,000 and I got it at 63,673 so I put 9,000 on it. I don't know any history of a rebuild in the past and am not experiencing any transmission problems.
My initial though was to service the clutch packs while I have it apart but Martin advised me not to disturb it unless I plan on a complete rebuild of the transmission.
So I am planning on just repairing the engine and going from there.

Any feedback on the cause of the piston failure is appreciated.

Robert

Attachments:

[wimfournier]

On a higher altitude you get leaner burning as a result of the lower air pressure. There fore you should retard the ignition to compensate. Leaner burning means higher end temperatures in the burning room above the piston. 
When you are driving in a environnement with higher altitude your car has had to work harder to get there than on flat roads. So enigne temp will have been higher too by that. Than it is not strange when one off the middle pistons gets too hot and breaks an edge above the pistonring by metal fatigue.

[69hcode]

I might have a standard piston or two out of my engine. It's an 81 as well. I know #4 is all knackered up. I'm swapping the whole thing for an 998 out of a 89 Japan car. 

[robertm]

Thanks for the offer 69hcode, I will send you a message once I get it stripped down and find out what it needs.

Oct 4, 2017 1:36:47 GMT 69hcode said:

I might have a standard piston or two out of my engine. It's an 81 as well. I know #4 is all knackered up. I'm swapping the whole thing for an 998 out of a 89 Japan car. 

[69hcode]

It's all yours. I'm not going to rebuild the 998 anyway. I have a 1275 SPI motor that will eventually get rebuilt. I'm going to run twin hs'2 on it. 

[robertm]

Update, I got my Mini back on the road. I took it out for its first test drive yesterday afternoon. The transmission shifted nicely while in Drive and in each individual gear selection. 

I actually had it started last Friday but the fuel pump sprung a leak so I had to wait on a replacement.
I was able to use a fuel bottle and gravity feed to run it for thirty minutes to check for leaks and such. I then drained the oil and gave it a fresh fill and a new filter.
I was surprised at the amount of debris and metal flake in that initial startup oil.

I'll probably get flamed and criticized for my low budget rebuild as I only replaced the pistons and cleaned up the bores with a ball hone. The original cylinder spec was 2.543 inches and I was getting measurements of 2.544 and 2.545 at the extreme.

As you can see from the picture, the number three piston had received thousands of tiny impacts dents and several larger impact dents.
It seemed that the top ring had eroded down to thin metal and began to break off and pieces had made their way into the combustion chamber. The cylinder head and valves looked relatively unharmed as compared to the piston.

I'll post another report if I have anymore trouble or if my cheap rebuild fails.

Robert       

[jockduck]

I think you are lucky if you did not get a score in the bore surface, I also had a piston/ring failure in my '68 998 a bit of broken ring gouged a big score in the bore surface, The rebore man had to rejig the bore centre to get a successful clean up. Number 4 cylinder from memory.
Jock

[richard1]

On a higher altitude you get leaner burning as a result of the lower air pressure. There fore you should retard the ignition to compensate. Leaner burning means higher end temperatures in the burning room above the piston.

Nope: With less air, the burn is slower, so you need to advance the timing to start the burn sooner. There are a couple of rules of thumb put there, but I just go by ear, and have been doing this going up and down between 600 ft and 15,000 feet for 50 years.

The general rules of thumb I read about suggest between 1/2º and 1º. I just turn a ¼ inch at a time and eventually get a good feel for it on a particular vehicle. My old Land Rover was great for this as the distributor had a knob to adjust up and down. I could set the bolt at one setting and adjust up or down from there with the thumb knob.

[notamini]

Our old A engines have the same thing on their original distributors. 4-6 clicks is enough to make the difference.

Mark